Method for forming apparatus comprising two dimensional material

ABSTRACT

A method and apparatus, the method comprising: forming at least two electrodes (23) on a release layer wherein the at least two electrodes are configured to enable a layer of two dimensional material (25) to be provided between the at least two electrodes; providing moldable polymer (27) overlaying the at least two electrodes; wherein the at least two electrodes and the moldable polymer form at least part of a planar surface (29).

TECHNOLOGICAL FIELD

Examples of the disclosure relate to a method for forming apparatuscomprising two dimensional material. In particular, they relate to amethod for forming electronic apparatus comprising two dimensionalmaterial such as graphene.

BACKGROUND

Apparatus comprising two dimensional materials such as graphene are wellknown. For instance graphene can be provided in devices such asresistive sensors or field effect transistors to enable parameters suchas chemicals or light to be detected. In other devices graphene fieldeffect transistors can be used as logic elements or other electroniccomponents.

It is useful to provide improved methods of forming such devices.

BRIEF SUMMARY

According to various, but not necessarily all, examples of thedisclosure there may be provided a method comprising: forming at leasttwo electrodes on a release layer wherein the at least two electrodesare configured to enable a layer of two dimensional material to beprovided between the at least two electrodes; providing mouldablepolymer overlaying the at least two electrodes; wherein the at least twoelectrodes and the mouldable polymer form at least part of a planarsurface.

In some examples the release layer may have a smooth surface to enable asmooth layer of two dimensional material to be provided.

In some examples the at least two electrodes may be provided in the sameplane.

In some examples the method may comprise providing the two dimensionalmaterial overlaying the electrodes after the electrodes have beenremoved from the release layer.

In some examples the method may comprise providing the two dimensionalmaterial on the release layer. The method may also comprise providing atleast part of the at least two electrodes overlaying the two dimensionalmaterial. The at least two electrodes, the two dimensional material andthe mouldable polymer may form at least part of the planar surface.

In some examples the method may comprise forming a composite polymersubstrate comprising the mouldable polymer. The method may also compriseproviding hard coating on the composite polymer substrate.

In some examples the two dimensional material and the at least twoelectrodes may form at least part of a bottom gate field effecttransistor.

In some examples the two dimensional material and the at least twoelectrodes form at least part of a top gate field effect transistor.

In some examples the method may comprise providing a plurality ofelectrodes and portions of two dimensional materials to form a pluralityof field effect transistors wherein at least some the field effecttransistors are bottom gate field effect transistors and at least someof the field effect transistors are top gate field effect transistors.

In some examples the two dimensional material may comprise graphene.

In some examples the method may comprise activating the two dimensionalmaterial.

In some examples the method may comprise activating the two dimensionalmaterial with quantum dots.

In some examples the mouldable polymer may provide a flexible substratefor the at least two electrodes after the at least two electrodes areremoved from the release layer.

In some examples the mouldable polymer may comprise at least one of,liquid polymer, mouldable polymer foil.

According to various, but not necessarily all, examples of thedisclosure there may be provided an apparatus formed by any of themethods described above.

According to various, but not necessarily all, examples of thedisclosure there may be provided an apparatus comprising: at least twoelectrodes and a layer of two dimensional material wherein the at leasttwo electrodes were formed on a release layer and the at least twoelectrodes are configured to enable the layer of two dimensionalmaterial to be provided between the at least two electrodes; andmouldable polymer overlaying the at least two electrodes; wherein the atleast two electrodes and the mouldable polymer form at least part of aplanar surface.

In some examples the release layer may be a smooth surface to enable asmooth layer of two dimensional material to be provided.

In some examples the at least two electrodes may be provided in the sameplane.

In some examples the two dimensional material may be provided overlayingthe electrodes after the electrodes have been removed from the releaselayer.

In some examples the two dimensional material may be provided on therelease layer. At least part of the at least two electrodes may beprovided overlaying the two dimensional material. The at least twoelectrodes, the two dimensional material and the mouldable polymer mayform at least part of the planar surface.

In some examples the apparatus may comprise a polymer substratecomprising the mouldable polymer. In some examples the apparatus maycomprise a hard coating on the composite polymer substrate.

In some examples the two dimensional material and the at least twoelectrodes may form at least part of a bottom gate field effecttransistor.

In some examples the two dimensional material and the at least twoelectrodes may form at least part of a top gate field effect transistor.

In some examples the apparatus may comprise a plurality of electrodesand portions of two dimensional materials which form a plurality offield effect transistors wherein at least some the field effecttransistors are bottom gate field effect transistors and at least someof the field effect transistors are top gate field effect transistors.

In some examples the two dimensional material may comprise graphene.

In some examples the two dimensional material may be activated.

In some examples the two dimensional material may be activated withquantum dots.

In some examples the mouldable polymer may provide a flexible substratefor the at least two electrodes after the at least two electrodes areremoved from the release layer

In some examples the mouldable polymer may comprise at least one of,liquid polymer, mouldable polymer foil.

According to various, but not necessarily all, examples of thedisclosure there is provided examples as claimed in the appended claims.

BRIEF DESCRIPTION

For a better understanding of various examples that are useful forunderstanding the detailed description, reference will now be made byway of example only to the accompanying drawings in which:

FIG. 1 illustrates a method;

FIG. 2 illustrates an apparatus;

FIGS. 3A to 3G illustrate an example method:

FIGS. 4A to 4I illustrate an example method;

FIGS. 5A to 5K illustrate an example method;

FIG. 6 illustrates an apparatus;

FIGS. 7A to 7K illustrate an example method;

FIG. 8 illustrates an apparatus; and

FIG. 9 illustrates an apparatus.

DETAILED DESCRIPTION

The Figures illustrate example methods and apparatus. The methods may beused to form apparatus comprising two dimensional material. Theapparatus may form electronic components within electronic devices. Insome examples the apparatus which are formed may be for sensing. Theapparatus may be for sensing environmental parameters such as light,temperature, chemicals or other parameters. The apparatus may be foractive sensing or for passive sensing. The apparatus may be aphotodetector and may be used for imaging. The apparatus may sense lightonly at a first wavelength or only within a first range of wavelengths.

FIG. 1 illustrates a method according to examples of the disclosure. Themethod may be used to form apparatus 21 comprising one or moreelectronic components where the electronic components comprise a twodimensional material such as graphene.

The method comprises, at block 11 forming at least two electrodes 23 ona release layer 33. The at least two electrodes 23 are configured toenable a layer of two dimensional material 25 to be provided between theat least two electrodes 23. The method also comprises, at block 13,providing mouldable polymer 27 overlaying the at least two electrodes23. The at least two electrodes 23 and the mouldable polymer 27 form atleast part of a planar surface 29.

It is to be appreciated that the electrodes 23 and the two dimensionalmaterial 25 may have any configuration which enables an electroniccomponent to be formed. Example methods for forming field effecttransistor (FET) devices are illustrated in more detail in FIGS. 3A to9. Other methods for forming other types of devices may be used in otherexamples of the disclosure.

FIG. 2 illustrates an example apparatus 21 which may be formed usingmethods such as the method of FIG. 1. The example apparatus 21 comprisesat least two electrodes 23 and a layer of two dimensional material 25.The at least two electrodes 23 were formed on a release layer 33. The atleast two electrodes 23 are configured to enable the layer of twodimensional material 25 to be provided between the at least twoelectrodes 23. The apparatus 21 also comprises mouldable polymer 27overlaying the electrodes 23. The at least two electrodes 23 and themouldable polymer 27 form at least part of a planar surface 29.

In the example of FIG. 2 the apparatus 21 comprises two electrodes 23.In this example the layer of two dimensional material 25 may be providedbetween the two electrodes 23 to form an electronic device such as aresistive sensor. It is to be appreciated that other arrangements of thelayer of two dimensional material 25 and the electrodes 23 may beprovided in other examples. For instance, in some examples the apparatus21 may comprise three electrodes 23 to enable FET devices to beprovided.

The electrodes 23 may comprise any suitable conductive material. Theelectrodes 23 may be electrically connected to the two dimensionalmaterial 25. The electrodes 23 may be electrically connected to the twodimensional material 25 to enable direct current to flow through theelectrodes 23 and the two dimensional material 25.

In the example of FIG. 2 both of the electrodes 23 are provided on thesame plane. Forming the electrodes 23 on the same release layer 33 mayensure that the electrodes 23 are provided within the same plane. Thisreduces the number of step edges in the apparatus 21.

The mouldable polymer 27 is provided overlaying the electrodes 23. Themouldable polymer 27 may be deposited overlaying the electrodes on therelease layer 33. The mouldable polymer 27 may comprise any polymermaterial which is fluid enough to embed the electrodes 23. Once themouldable polymer 27 is provided around the electrodes 23 the mouldablepolymer 27 may be cured or otherwise hardened. Once the mouldablepolymer 27 has hardened it may form a flexible substrate for the atleast two electrodes 23. The mouldable polymer 27 may form a thinflexible substrate.

Depositing the mouldable polymer 27 on the same release layer as theelectrodes 23 may enable the mouldable polymer 27 and the electrodes 23to form at least part of a planar surface 29. The planar surface 29 maycomprise a smooth flat surface. The release layer 33 may comprise amaterial having a smooth flat surface to ensure that the planar surface29 is also smooth and flat. The other electronic components of theapparatus 21 such as the layer of two dimensional material 25 orelectrical connections to the layer of two dimensional material 25 maybe deposited on the planar surface 29.

The layer of two dimensional material 25 may comprise a very thin layerof material. In some examples the layer of two dimensional material 25could be an atomic monolayer. In some examples the layer of twodimensional material 25 could comprise several atomic monolayers. Thelayer of two dimensional material 25 could comprise graphene, molybdenumdisulphide, boron nitride or any other suitable material.

In the example apparatus 21 of FIG. 2 the layer of two dimensionalmaterial 25 is provided overlaying at least part of the electrodes 23.The two dimensional material 25 may be provided overlaying theelectrodes 23 after the electrodes 23 have been removed from the releaselayer 33. In other examples the two dimensional material 25 could alsobe formed or deposited on the release layer 33 along with the electrodes23.

The layer of two dimensional material 25 is provided on the planarsurface 29. As a smooth flat surface is provided for the two dimensionalmaterial 25 this reduces the amount of discontinuities and/or impuritiesin the two dimensional material 25 and may provide for improved chargetransfer characteristics of the two dimensional material 25.

FIGS. 3A to 3G illustrate example methods which may be used to formother example apparatus 21. The example method of FIGS. 3A to 3Gcomprises forming a mouldable polymer 27 substrate with a plurality ofembedded electrodes 23. The mouldable polymer 27 and the embeddedelectrodes 23 form a planar surface 29 which can then be used fordepositing graphene or any other suitable two dimensional material 25.

In FIG. 3A a release layer 33 is provided on a carrier substrate 31. Inthe example of FIG. 3A the carrier substrate 31 may provide a rigid orsubstantially rigid substrate which may provide support while theelectrodes 23 and/or other components of the apparatus 21 are beingfabricated on the release layer 33. The carrier substrate 31 maycomprise a silicon wafer or any other suitable material.

The carrier substrate 31 may be flat or substantially flat.

The release layer 33 is provided overlaying the carrier substrate 31.The release layer 33 may comprise a sacrificial layer which may enablethe components of the apparatus 21 that are fabricated to be removedfrom the carrier substrate 31. The material that is used for the releaselayer 33 may depend on the components that are being fabricated and thematerial that is being used for those components. In some examples therelease layer 33 may comprise copper or any other suitable material.

The release layer 33 may have a smooth surface 32. The components of theapparatus 21 may be formed on the smooth surface 32 of the release layer33 so that the components of the apparatus 21 form a planar surface 29.The release layer 33 has a surface which is smooth enough to enable asmooth layer of two dimensional material 25 to be provided. The twodimensional material 25 could be provided on the release layer 33 or ona planar surface 29 which has been formed on the release layer 33.

In FIG. 3A the electrodes 23 are deposited on the release layer 33. Inthe example of FIG. 3A three electrodes 23 are provided. The threeelectrodes 23 may form a source, gate and drain electrode for an FET.Each of the electrodes 23 are provided in the same plane. This reducesthe number of step edges in the apparatus 21. It is to be appreciatedthat other arrangements of electrodes may be used in other examples ofthe disclosure.

The electrodes 23 may comprise any conductive material such as a metal.

The electrodes 23 may be deposited using any suitable technique. Forinstance the electrodes 23 could be formed by photolithography followedby thermal or electron beam evaporation of a metal, or any othersuitable process.

In FIG. 3B mouldable polymer 27 is provided overlaying the electrodes23. The mouldable polymer 27 is deposited on the release layer 33overlaying the electrodes 23. The mouldable polymer 27 comprises anypolymer which will embed the electrodes 23 and form a planar surface 29against the surface 32 of the release layer 33.

In some examples the mouldable polymer 27 may comprise a liquid polymerwhich may be deposited onto the release layer 33 via spin coating, spraycoating or any other suitable process. In other examples the mouldablepolymer 27 may comprise a polymer foil which may be deposited by hotembossing or any other suitable process.

In FIG. 3C the carrier substrate 31 and release layer 33 are removed.The mouldable polymer 27 may be hardened or cured before the carriersubstrate 31 and release layer 33 are removed so that the mouldablepolymer 27 provides a substrate for the electrodes 23. The mouldablepolymer 27 may provide a flexible substrate for the electrodes 23. Themouldable polymer 27 may enable further components of the apparatus 21to be fabricated overlaying the electrodes 23.

The mouldable polymer 27 and the electrodes 23 form a planar surface 29.The planar surface 29 may be smooth and flat. The planar surface 29 maybe a uniform or substantially uniform surface.

The other components of the apparatus 21 may be fabricated on the planarsurface 29 formed by the mouldable polymer 27 and the electrodes 23. InFIG. 3D a dielectric 35 is provided on the planar surface 29. In theexample of FIG. 3D the dielectric 35 is provided overlaying the gateelectrode 23 and at least part of the source and drain electrodes 23.

The dielectric 35 may comprise any suitable insulating material. In someexamples the dielectric 35 may comprise aluminum oxide which could bedeposited using atomic layer deposition or any other suitable process.The dielectric 35 may be provided in a thin layer.

In FIG. 3E a layer of two dimensional material 25 is deposited on to theplanar surface 29. In the example of FIGS. 3A to 3G the two dimensionalmaterial 25 comprises graphene.

The graphene may be deposited on to the planar surface 29 using anysuitable technique. In some examples the graphene may be formed on aseparate substrate and transferred onto the planar surface 29. Thegraphene may then be patterned using photolithography, plasma etching orany other suitable process.

In the example of FIG. 3E the graphene is provided overlaying thedielectric 35 so that the dielectric 35 forms an insulating barrierbetween the graphene and the electrodes 23.

In FIG. 3F contacts 37 are provided between the source and drainelectrodes 23 and the graphene. The contacts 37 may provide a directcurrent path between the source and drain electrodes 23 and thegraphene. The contacts 37 may comprise any conductive material, such asa metal, which may be deposited between the electrodes 23 and thegraphene. The contacts 37 may be deposited using photolithography, metalevaporation or any other suitable process.

In FIG. 3G the graphene is activated. The activation of the graphene mayenable the FET to be used as a sensor. The material that is used toactivate the graphene may depend on the parameters that the FET isintended to detect. In the example of FIG. 3G the graphene is activatedwith quantum dots 39. The quantum dots 39 may be deposited using anysuitable technique such as spin coating, inkjet printing, wet transferor any other suitable process.

It is to be appreciated that variations of the method of FIGS. 3A to 3Gmay be made in other examples of the disclosure. For instance, in theexamples of FIGS. 3A to 3G the dielectric 35 is formed on the planarsurface 29 after the electrode 23 and the mouldable polymer 27 have beenremoved from the release layer 33. In other examples the dielectric 35could be formed on the release layer 33. In such examples the mouldablepolymer 27 would then be deposited overlaying both the dielectric 35 andthe electrodes 23. This would enable the planar surface 29 to be formedfrom the mouldable polymer 27, the electrodes 23 and the dielectric 35.The graphene, or other two dimensional material 25, could then bedeposited on the planar surface 29. This method avoids the introductionof any step edges in the connection between the electrodes 23 and thegraphene. This method may be useful in apparatus 21 where the graphenelayer is larger than the dielectric 35 layer.

FIGS. 4A to 4I illustrate another example method which may be used toform other example apparatus 21. In the example of FIGS. 4A to 4I thetwo dimensional material 25 is deposited on the release layer 33. Inthis example the mouldable polymer 27, the two dimensional material 25and the embedded electrodes 23 form a planar surface 29. This may enablethe two dimensional material 25 to be provided in the same plane as theelectrodes 23.

In FIG. 4A a carrier substrate 31 is provided. The carrier substrate 31may provide a rigid or substantially rigid substrate as described above.

In FIG. 4B the release layer 33 is provided overlaying the carriersubstrate 31. The release layer 33 may comprise a sacrificial layer witha smooth surface which may be as described above. In the examples ofFIGS. 4A to 4I the release layer 33 may comprise a material such ascopper which may enable graphene to be deposited onto it.

In FIG. 4C a layer of two dimensional material 25 is deposited onto therelease layer 33. In the example of FIGS. 4A to 4I the two dimensionalmaterial 25 comprises graphene. Other two dimensional materials may beused in other examples of the disclosure.

The graphene may be deposited on the release layer 33 using chemicalvapour deposition, a wet transfer process, a dry transfer process or anyother suitable process. The graphene may be patterned on the releaselayer 33 in order to provide the correct channel dimensions for theapparatus 21.

In FIG. 4D a dielectric 35 is deposited over the graphene. Thedielectric 35 may be deposited so that it completely covers thegraphene. In the example of FIG. 4D, where the graphene is stillattached to the release layer 33, the release layer 33 and thedielectric 35 completely envelop the graphene.

The dielectric 35 may comprise any suitable insulating material. In someexamples the dielectric 35 may comprise aluminum oxide which could bedeposited using atomic layer deposition or any other suitable process.The dielectric 35 may be provided in a thin layer.

In some examples the graphene may be activated before the dielectric 35is deposited. The activation of the graphene may counteract the lowsurface energy of the graphene and may enable uniform deposition of thedielectric 35 over the graphene. For instance, a seed layer may beevaporated onto the graphene to enable the atomic layer deposition.

In FIG. 4E a plurality of electrodes 23 are deposited onto the releaselayer 33. In the example of FIGS. 4A to 4I the electrodes 23 comprisesource, drain and gate electrodes. The electrodes 23 are deposited sothat at least part of the electrodes 23 extend over the dielectric 35.The source and drain electrodes 23 are deposited so that at least partof the source and drain electrodes 23 are in direct contact with thesurface 32 of the release layer 33. The electrodes 23 are formed so thatat least the source and drain electrodes 23 are in the same plane as thegraphene. This reduces the number of step edges in the apparatus 21.

The electrodes 23 may be formed using any suitable technique. Forinstance, in some examples the electrodes 23 may be formed byphotolithography followed by evaporation of the electrode material.

In FIG. 4F mouldable polymer 27 is provided overlaying the electrodes23, dielectric 35 and graphene. The mouldable polymer 27 is deposited onthe release layer 33 overlaying the electrodes 23, dielectric 35 andgraphene. The mouldable polymer 27 comprises any polymer which willembed the electrodes 23, dielectric 35 and graphene and form a planarsurface 29 against the surface 32 of the release layer 33.

In some examples the mouldable polymer 27 may comprise a liquid polymerwhich may be deposited onto the release layer 33 via spin coating, spraycoating or any other suitable process. In other examples the mouldablepolymer 27 may comprise a polymer foil which may be deposited by hotembossing or any other suitable process.

In FIG. 4G the carrier substrate 31 and release layer 33 are removed.The mouldable polymer 27 may be hardened or cured before the carriersubstrate 31 and release layer 33 are removed so that the mouldablepolymer 27 provides a substrate for the electrodes 23 and the graphene.

Once the release layer 33 has been removed the mouldable polymer 27, thegraphene and the electrodes 23 form a planar surface 29. The planarsurface 29 may be smooth and flat. The planar surface 29 may be auniform or substantially uniform surface.

The other components of the apparatus 21 may be fabricated on the planarsurface 29 formed by the mouldable polymer 27, the graphene and theelectrodes 23.

In FIG. 4H contacts 37 are provided between the source and drainelectrodes 23 and the graphene. The contacts 37 may be provided on theplanar surface 29. The contacts 37 may provide a direct current pathbetween the source and drain electrodes 23 and the graphene. Thecontacts 37 may comprise any conductive material, such as a metal, whichmay be deposited between the electrodes 23 and the graphene. Thecontacts 37 may be deposited using photolithography, metal evaporationor any other suitable process. The contacts 37 may form a FET.

In FIG. 4I the graphene is activated. In the example of FIG. 4I thegraphene is activated with quantum dots 39. The activation of thegraphene may enable the FET to be used as a sensor. The material that isused to activate the graphene may depend on the parameters that the FETis intended to detect. In some examples the graphene might not beactivated.

FIGS. 5A to 5K illustrate an example method which may be used to formother example apparatus 21. The example method of FIGS. 5A to 5K may beused to form apparatus 21 comprising bottom gate FET devices.

In FIG. 5A a carrier substrate 31 and a release layer 33 are provided.The carrier substrate 31 may comprise silicon, as described above, orany other suitable material. The release layer 33 is provided overlayingthe carrier substrate 31. The release layer 33 may comprise asacrificial layer with a smooth surface which may also be as describedabove. The release layer 33 has a smooth surface 32 on which componentsof an apparatus 21 can be fabricated.

In FIG. 5B a plurality of electrodes 23 are provided. The plurality ofelectrodes 23 form the source, gate and drain electrodes 23 of a bottomgate FET device. The plurality of electrodes 23 are deposited on thesmooth surface 32 of the release layer. As the electrodes 23 are formedon the same smooth surface 32 all of the electrodes 23 are provided inthe same plane. This reduces the number of step edges in the apparatus21.

In FIGS. 5C and 5D a composite polymer substrate 53 is formed to supportthe plurality of electrodes 23. The composite polymer substrate 53supports the electrodes 23 after they have been removed from the releaselayer 33.

In the examples of FIGS. 5A to 5K the composite polymer substrate 53comprises two different polymers. It is to be appreciated that in someexamples the composite polymer substrate 53 could comprise more than twodifferent polymers. The at least two polymers are laminated together toform a single polymer substrate 53.

In the examples of FIGS. 5A to 5K the composite polymer substrate 53 isformed from a mouldable polymer 27 and a polymer foil 51. The mouldablepolymer 27 may comprise any suitable material which will embed theelectrodes 23 and form a planar surface 29 against the surface 32 of therelease layer 33. The mouldable polymer 27 may comprise a thermosettingor ultra violet (UV) curable resin. This may enable the mouldablepolymer 27 to be solidified after it has been deposited over theelectrodes 23.

The mouldable polymer 27 may comprise a polymer resin which has aviscosity which enables the mouldable polymer 27 to embed the electrodes23. In some examples the mouldable polymer 27 may have a viscosity ofbetween 5 cP to 500 cP.

The mouldable polymer 27 may comprise a material which enables certainparameters to pass through. For instance, in the example of FIGS. 5A to5K the apparatus 21 may be used as a photodetector. In such examples themouldable polymer 27 may be transparent or at least partiallytransparent to visible light. It is to be appreciated that the apparatus21 could be configured to sense other parameters in other examples ofthe disclosure.

The mouldable polymer 27 can be deposited on either the release layer 33or the polymer foil 51 using any suitable technique. For instance themouldable polymer 27 may be deposited using spin coating, bar coating,slot-die coating or any other suitable process.

After the moldable polymer 27 has been cured the moldable polymer 27 mayform a thin layer. The thickness of the layer of moldable polymer 27 maybe controlled by the thickness of the layer or moldable polymer 27 whichis applied, the pressure applied to the apparatus 21 and the rheologicalproperties of the moldable polymer 27. In some examples the thickness ofthe layer of moldable polymer 27 could be between 50 nm and 10 μm.

The mouldable polymer 27 is provided directly overlaying the electrodes.The mouldable polymer 27 is provided on the release layer 33 overlayingthe electrodes 23.

The polymer foil 51 is provided overlaying the mouldable polymer 27. Inthe example of FIGS. 5A to 5K the mouldable polymer 27 is providedbetween the polymer foil 51 and the release layer 33 so that the polymerfoil 51 does not directly contact the surface 32 of the release layer33.

The polymer foil 51 may comprise a solid polymer. In some example thepolymer foil 51 may comprise a flexible polymer which may deform when auser applies a force to the apparatus. The polymer foil 51 may comprisea polymer material which enables certain parameters to pass through. Inexamples where the apparatus 21 is used as a photodetector the polymerfoil 51 may be arranged to be transparent to visible light.

For instance the polymer foil 51 may comprise a material such aspolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyethersulfone (PES) or any other suitable material. Such materialsmay enable 90% or more of incident visibly light to pass through thepolymer foil 51.

The polymer foil 51 may have a greater thickness than the layer ofmouldable polymer 27. In some examples the thickness of the polymer foil51 could be between 10 μm to 1000 μm.

FIG. 5C shows two different examples of depositing the mouldable polymer27 and the polymer foil 51. In the first example the mouldable polymer27 and the polymer foil 51 are deposited separately. In such examplesthe mouldable polymer 27 is provided overlaying the electrodes 23 andthen the polymer foil 51 is provided overlaying the mouldable polymer27.

In the second example the mouldable polymer 27 and the polymer foil 51are deposited at the same time. In such examples the mouldable polymer27 may be adhered to the underside of the polymer foil 51. Both themouldable polymer 27 and the polymer foil 51 are then providedoverlaying the electrodes 23.

In some examples the surfaces of different layers within the compositepolymer substrate 53 may be treated to improve the adhesion between therespective layers. In some examples surface activation techniques suchas plasma, corona treatments, ultraviolet/ozone (UVO) or any othersuitable process could be used. In some examples adhesion promoters suchas primers, self-assembled monolayers (SAM), copolymers or any othersuitable material may be used.

In FIG. 5D the composite polymer substrate 53 is cured. In the exampleof FIG. 5D the composite polymer substrate 53 is cured using UV light.It is to be appreciated that other means of curing may be used in otherexamples of the disclosure. For instance in some examples the mouldablepolymer 27 may comprise a thermosetting resin. In such examples thecuring may comprise heating the mouldable polymer 27. The temperature towhich the mouldable polymer 27 is heated may depend on the materialwhich is used. In some examples the mouldable polymer 27 may be heatedto a temperature of around 200° C.

The cured moldable polymer 27 may have a low coefficient of thermalexpansion. This may prevent deformation of the apparatus 21 and ensurethat the electrodes 23 remain within the same plane.

The cured moldable polymer 27 and the polymer foil 51 may have similarmechanical properties to reduce stresses and deformations within anapparatus 21. In some examples the elastic modulus and/or coefficient ofthermal expansion of the cured moldable polymer 27 and the polymer foil51 may be similar.

FIG. 5D shows two different examples of the composite polymer substrate53. In the first example the polymer foil 51 has no additional coating.In the second example a hard coating 55 is provided on the polymer foil51. In the example of FIG. 5D the hard coating 55 is provided on twosides of the polymer foil 51. In other examples the hard coating 55might only be provided on one side.

The hard coating 55 may be configured to provide a barrier layer toprevent contamination of the electronic components of the apparatus 21.For instance the hard coating may prevent the ingress of oxygen,moisture or other contaminants.

In some examples the hard coating 55 may be configured to improve theabsorption of a parameter which the apparatus 21 is intended to detect.For instance, where the apparatus 21 is arranged to detect visible lightthe hard coating 55 may comprise an antireflective coating which mayimprove the penetration of light into the apparatus 21.

In some examples the hard coating 55 may comprise a nanoscale coating.The nanoscale coating may comprise a material such as SiO_(x), SiN_(x),AlO_(x), AlN_(x). the hard coating 55 may be deposited on the polymerfoil 51 using any suitable technique.

For instance the hard coating 55 could be deposited by atomic layerdeposition, plasma enhanced chemical vapour deposition or any othersuitable process.

In FIG. 5E the carrier substrate 31 and release layer 33 are removed sothat the mouldable polymer 27 and the polymer foil 51 provide acomposite substrate 53 for the electrodes 23.

The mouldable polymer 27 and the electrodes 23 form a planar surface 29.The planar surface 29 may be smooth and flat. The planar surface 29 maybe a uniform or substantially uniform surface.

The other components of the apparatus 21 may be fabricated on the planarsurface 29 formed by the mouldable polymer 27 and the electrodes 23. InFIG. 5F a dielectric 35 is provided on the planar surface 29. In theexample of FIG. 3D the dielectric 35 is provided overlaying theelectrodes 23.

The dielectric 35 may comprise any suitable insulating material. In someexamples the dielectric 35 may comprise an inorganic oxide or nitridewhich could be deposited using atomic layer deposition. In otherexamples the dielectric 35 may comprise an organic polymer which couldbe deposited by a coating or printing method. The dielectric 35 may beprovided in a thin layer.

In FIG. 5G a layer of two dimensional material 25 is deposited on to theplanar surface 29. In the example of FIGS. 5A to 5K the two dimensionalmaterial 25 comprises graphene.

The graphene may be deposited on to the planar surface 29 using anysuitable technique. In some examples the graphene may comprise amonolayer which may be formed by chemical vapor deposition on a metalfoil or any other suitable technique. The graphene monolayer may then betransferred onto the planar surface 29 using a transfer substrate, suchas a poly(methyl methacrylate) (PMMA) substrate, or any other suitableprocess.

In the example of FIG. 5G the graphene is provided overlaying thedielectric 35 so that the dielectric 35 forms an insulating barrierbetween the graphene and the embedded electrodes 23.

Both the dielectric 35 and the graphene are formed on the planar surface29 formed by the mouldable polymer 27 and the embedded electrodes 23.This allows the dielectric 35 and the graphene to be formed without anysteps or discontinuities. This reduces the structural defects within thegraphene and improves the electrical characteristics of the apparatus21.

In FIG. 5H the graphene and the dielectric 35 are patterned. Thegraphene and the dielectric 35 may be patterned into any suitable shape.In the example of FIGS. 5A to 5K the graphene and the dielectric 35 maybe patterned to enable an FET to be formed. In the example of FIG. 5Hthe graphene and the dielectric 35 are patterned so that at least partof the source and drain electrodes 23 are uncovered.

In FIG. 5I contacts 37 are provided between the source and drainelectrodes 23 and the graphene. The contacts 37 may provide a directcurrent path between the source and drain electrodes 23 and thegraphene. The contacts 37 may comprise any conductive material, such asa metal, which may be deposited between the electrodes 23 and thegraphene. The contacts 37 may be deposited using photolithography, metalevaporation or any other suitable process.

In FIG. 5J the graphene is activated. The activation of the graphene mayenable the FET to be used as a sensor. The material that is used toactivate the graphene may depend on the parameters that the FET isintended to detect. In the example of FIG. 5J the graphene is activatedwith quantum dots 39. The quantum dots 39 may be deposited using anysuitable technique such as spin coating, inkjet printing, wet transferor any other suitable process.

In FIG. 5K an encapsulating layer is provided on the planer surface 29.The encapsulating layer 57 is provided overlaying the graphene and thecontacts 37. The encapsulating layer 57 may protect the apparatus 21from contaminants such as moisture, oxygen or other chemicals. Theencapsulating layer 57 may be transparent to the parameter that theapparatus 21 is intended to detect. For instance, where the apparatus 21is arranged to detect visible light the encapsulating layer 57 may betransparent to visible light.

The methods of FIGS. 5A to 5K enable an apparatus 21 comprising a bottomgate GFET to be formed. FIG. 6 illustrates an example apparatus 21 whichhas been formed by the method of FIG. 5A to 5K. In the example apparatus21 the bottom gate GFET (graphene field effect transistor) is configuredto act as a photodetector. The apparatus 21 is configured so thatphotons 61 which are incident on the apparatus 21 can pass through theencapsulating layer 57 and/or the polymer foil 51 and may be incident onthe GFET.

In some examples the composite polymer substrate 53 may be arranged toact as a light filter. In such examples the composite polymer substrate53 may comprise one or more polymer layers which is transparent to lightin a first range of wavelengths but blocks light outside of the firstrange of wavelengths.

FIGS. 7A to 7K illustrate an example method which may be used to formother example apparatus 21. The example method of FIGS. 7A to 7K may beused to form apparatus 21 comprising top gate FET devices.

In FIG. 7A a carrier substrate 31 and a release layer 33 are provided.The carrier substrate 31 may comprise silicon, as described above, orany other suitable material. The release layer 33 is provided overlayingthe carrier substrate 31. The release layer 33 may comprise asacrificial layer with a smooth surface which may also be as describedabove. The release layer 33 has a smooth surface 32 on which componentsof an apparatus 21 can be fabricated.

In FIG. 7B a plurality of electrodes 23 are provided. In the example ofFIG. 7B two electrodes 23 are provided. The plurality of electrodes 23form the source and drain electrodes 23 of a top gate FET device. Theplurality of electrodes 23 are deposited on the smooth surface 32 of therelease layer. As the electrodes 23 are formed on the same smoothsurface 32 all of the electrodes 23 are provided in the same plane. Thisreduces the number of step edges in the apparatus 21.

In FIG. 7C quantum dots 39 are provided on the surface 32 of the releaselayer 33. The quantum dots 39 may be deposited using any suitabletechnique such as spin coating, inkjet printing, wet transfer or anyother suitable process. The quantum dots 39 are provided between thesource and drain electrodes 23 and may be arranged to activate graphenewhich can be deposited in another block of the method.

In FIGS. 7D and 7E a composite polymer substrate 53 is formed to supportthe plurality of electrodes 23. The composite polymer substrate 53 maybe as described above in relation FIGS. 5A to 5K. In the example ofFIGS. 7D and 7E the composite polymer substrate 53 embeds both theelectrodes 23 and the quantum dots 39.

In FIG. 7F the carrier substrate 31 and release layer 33 are removed sothat the mouldable polymer 27 and the polymer foil 51 provide acomposite substrate 53 for the electrodes 23 and the quantum dots 39.

The mouldable polymer 27 and the electrodes 23 form a planar surface 29.The planar surface 29 may be smooth and flat. The planar surface 29 maybe a uniform or substantially uniform surface. The smooth flat surfacemay reduce the discontinuities and irregularities in the graphene thatis deposited on the planar surface 29.

In FIG. 7G a layer of two dimensional material 25 is deposited on to theplanar surface 29. In the example of FIGS. 7A to 7K the two dimensionalmaterial 25 comprises graphene.

The graphene may be deposited on to the planar surface 29 using anysuitable technique. In some examples the graphene may comprise amonolayer which may be formed by chemical vapor deposition on a metalfoil or any other suitable technique. The graphene monolayer may then betransferred onto the planar surface 29 using a transfer substrate, suchas a poly(methyl methacrylate) (PMMA) substrate, or any other suitableprocess.

In the example of FIG. 7G the graphene is provided overlaying theelectrodes 23 and the quantum dots 39.

In FIG. 7H a dielectric 35 is provided overlaying the graphene. Thedielectric 35 may comprise any suitable insulating material. In someexamples the dielectric 35 may comprise an inorganic oxide or nitridewhich could be deposited using atomic layer deposition. In otherexamples the dielectric 35 may comprise an organic polymer which couldbe deposited by a coating or printing method. The dielectric 35 may beprovided in a thin layer.

Both the dielectric 35 and the graphene are formed on the planar surface29 formed by the mouldable polymer 27 and the embedded electrodes 23.This allows the dielectric 35 and the graphene to be formed without anysteps or discontinuities.

This reduces the structural defects within the graphene and improves theelectrical characteristics of the apparatus 21.

In FIG. 7I the graphene and the dielectric 35 are patterned. Thegraphene and the dielectric 35 may be patterned into any suitable shape.In the example of FIGS. 7A to 7K the graphene and the dielectric 35 maybe patterned to enable an FET to be formed. In the example of FIG. 7Ithe graphene and the dielectric 35 are patterned so that at least partof the source and drain electrodes 23 are uncovered.

In FIG. 7J a gate electrode 23 is provided overlaying the dielectric 35.In the example of FIG. 7J the gate electrode 23 is provided overlayingthe dielectric 35 so that the dielectric 35 forms an insulating barrierbetween the graphene and the gate electrode 23.

The gate electrode 23 may comprise any conductive material, such as ametal, which may be deposited overlaying the dielectric 35. The contacts37 may be deposited using photolithography, metal evaporation or anyother suitable process.

In FIG. 7K an encapsulating layer is provided on the planer surface 29.The encapsulating layer 57 is provided overlaying the graphene and thecontacts 37. The encapsulating layer 57 may protect the apparatus 21from contaminants such as moisture, oxygen or other chemicals. Theencapsulating layer 57 may be transparent to the parameter that theapparatus 21 is intended to detect. For instance, where the apparatus 21is arranged to detect visible light the encapsulating layer 57 may betransparent to visible light.

The methods of FIGS. 7A to 7K enable an apparatus 21 comprising a topgate GFET to be formed. FIG. 8 illustrates an example apparatus 21 whichhas been formed by the method of FIG. 7A to 7K. In the example apparatus21 the top gate GFET (graphene field effect transistor) is configured toact as a photodetector. The apparatus 21 is configured so that photons61 which are incident on the apparatus 21 can pass through theencapsulating layer 57 and/or the polymer foil 51 and may be incident onthe GFET.

FIG. 9 illustrates an apparatus 21 which comprises top gate GFETs andbottom gate GFETs. The example methods of FIGS. 5A to 5K and 7A to 7Kmay be performed using the same composite substrate 53 to form anapparatus 21 such as the apparatus of FIG. 9. This enables a doublesided sensor to be provided at low cost.

In the example of FIG. 9 only one top gate GFET and one bottom gate GFETis illustrated. It is to be appreciated that any number of top gateGFETs bottom gate GFETs may be provided in other examples. In someexamples a plurality of apparatus might be provided in an array. Some ofthe apparatus 21 may comprise top gate GFETs and some of the apparatus21 may comprise bottom gate GFETs.

In the example of FIG. 9 both of the GFETs are arranged asphotodetectors. It is to be appreciated that in some examples thedifferent apparatus 21 could be arranged to detect different parameters.For instance a first GFET could be configured as a photodetector andanother GFET could be configured to detect moisture or other chemicals.

Examples of the disclosure provide methods of forming apparatus 21comprising two or more coplanar electrodes 23 and a channel of twodimensional material 25. Having at least the two electrodes 23 in thesame plane reduces the number of steps or other discontinuities in thetwo dimensional material 25 which reduces the number of defects withinthe two dimensional material 25. Reducing the number of defects withinthe two dimensional material 25 increases carrier mobility within thechannel of two dimensional material 25 and provides for an improvedapparatus 21.

Examples of the disclosure also provide smooth flat surfaces for thedeposition of graphene or other two dimensional material 25. Having asmooth flat surface reduces a number of factors which can reduce thecarrier mobility in the two dimensional material such as defects in thetwo dimensional material 25, contamination of the two dimensionalmaterial 25, charge concentrations in the substrate supporting the twodimensional material 25, water or other contaminants trapped between thetwo dimensional material 25 and the substrate and other similar factors.Having a smooth flat surface for the deposition of graphene or other twodimensional material 25 also allows for good contact between the twodimensional material 25 and dielectric 35 or electrode 23.

In some examples the embedding of components such as electrodes 23, twodimensional material 25 and dielectric 35 can be used to control theposition of the components relative to the neutral plane. As theapparatus 21 can be very thin the components of the apparatus 21 can bepositioned very close to the neutral axis of the apparatus 21. This mayprovide for a more resilient apparatus 21 and may enable strainsensitive components to be protected when the apparatus 21 is bent orotherwise deformed. This may also enable the apparatus 21 to be bent toa higher degree of curvature.

Examples of the disclosure which use a composite polymer substrate 53may provide for improved transparency to parameters such as visiblelight. As the mouldable polymer 27 is adhered to a polymer foil 51 toprovide a composite substrate only a thin layer of the mouldable polymer27 is needed. The polymer foil 51 may comprise a material which istransparent to a parameter which is to be detected by the apparatus 21.This allows for both transparency and mechanical flexibility.

The use of a composite polymer substrate 53 enables different polymersto be used for different apparatus 21. This allows the polymers to bechosen to address the requirements of the apparatus 21 that is beingformed and/or the parameters that the apparatus 21 is intended todetect.

The methods of the disclosure may enable large numbers of apparatus 21to be produced at low costs. The method may be fast as processes such ascuring may only take several seconds to be completed. The method mayavoid the use of high temperatures which could damage sensitivecomponents. For instance the thermosetting resins may be set attemperatures of 200° C. which may be low enough to avoid damaging othercomponents of the apparatus 21.

In the above description the term “coupled” means operationally coupled.Any number of intervening components may be provided including nointervening components.

The term “comprise” is used in this document with an inclusive not anexclusive meaning. That is any reference to X comprising Y indicatesthat X may comprise only one Y or may comprise more than one Y. If it isintended to use “comprise” with an exclusive meaning then it will bemade clear in the context by referring to “comprising only one . . . ”or by using “consisting”.

In this brief description, reference has been made to various examples.The description of features or functions in relation to an exampleindicates that those features or functions are present in that example.The use of the term “example” or “for example” or “may” in the textdenotes, whether explicitly stated or not, that such features orfunctions are present in at least the described example, whetherdescribed as an example or not, and that they can be, but are notnecessarily, present in some of or all other examples. Thus “example”,“for example” or “may” refers to a particular instance in a class ofexamples. A property of the instance can be a property of only thatinstance or a property of the class or a property of a sub-class of theclass that includes some but not all of the instances in the class. Itis therefore implicitly disclosed that a features described withreference to one example but not with reference to another example, canwhere possible be used in that other example but does not necessarilyhave to be used in that other example.

Although embodiments of the present invention have been described in thepreceding paragraphs with reference to various examples, it should beappreciated that modifications to the examples given can be made withoutdeparting from the scope of the invention as claimed.

Features described in the preceding description may be used incombinations other than the combinations explicitly described.

Although functions have been described with reference to certainfeatures, those functions may be performable by other features whetherdescribed or not.

Although features have been described with reference to certainembodiments, those features may also be present in other embodimentswhether described or not.

Whilst endeavoring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

We claim:
 1. A method for forming a field effect transistor, said method comprising: providing a release layer with a smooth surface on a carrier substrate; depositing source, gate and drain electrodes on the release layer; depositing a mouldable polymer overlaying the source, gate and drain electrodes on the release layer, so that the source, gate and drain electrodes and the mouldable polymer form a planar surface against the smooth surface of the release layer; removing the carrier substrate and the release layer; providing a dielectric on the planar surface overlying the gate electrode and at least part of the source and drain electrodes; depositing a layer of two dimensional material on the dielectric; providing a first contact between the source electrode and the two dimensional material so that the contact provides a direct current path between the source electrode and the two dimensional material; and providing a second contact between the drain electrode and the two dimensional material so that the contact provides a direct current path between the drain electrode and the two dimensional material.
 2. The method according to claim 1, wherein the two dimensional material comprises graphene.
 3. The method according to claim 1, wherein the two-dimensional material is activated with quantum dots.
 4. The method according to claim 1, wherein, before the contacts are provided, the two-dimensional material and the dielectric are patterned so that at least part of the source and drain electrodes are uncovered.
 5. The method according to claim 1, further comprising providing a plurality of electrodes and portions of two dimensional materials to form a plurality of field effect transistors, wherein at least some the field effect transistors are bottom gate field effect transistors and at least some of the field effect transistors are top gate field effect transistors.
 6. A method for forming a field effect transistor, said method comprising: providing a release layer with a smooth surface on a carrier substrate; depositing source and drain electrodes on the smooth surface of the release layer; depositing quantum dots between the source and drain electrodes on the smooth surface of the release layer; depositing a mouldable polymer overlaying the source and drain electrodes on the release layer, so that the source and drain electrodes and the mouldable polymer form a planar surface against the smooth surface of the release layer; removing the carrier substrate and the release layer; depositing a layer of two dimensional material on the planar surface; providing a dielectric on the two dimensional material; patterning the two-dimensional material and the dielectric so that at least part of the source and drain electrodes are uncovered; and providing a gate electrode overlaying the dielectric.
 7. The method according to claim 6, wherein the two dimensional material comprises graphene.
 8. The method according to claim 6, further comprising providing a plurality of electrodes and portions of two dimensional materials to form a plurality of field effect transistors wherein at least some the field effect transistors are bottom gate field effect transistors and at least some of the field effect transistors are top gate field effect transistors. 